Mlecular sieve zeolites have long been observed to demonstrate selective adsorption when in contact with a variety of adsorbable mixtures. This attribute may be utilized to affect a variety of separations, as for example, the separation of n-paraffins from branched chain paraffins or other well known separations using pressure swing or vacuum swing processes. The adsorptive selectivity of the zeolite towards one or more components of a mixture must be maximized to maximize the efficiency of the desired separation. Assuming all other engineering factors remain constant, the adsorption characteristics of the material selected for the separation process influences both the production level and the purity of the gases produced.
The phenomenon of selective adsorption by molecular sieve zeolites may arise from one of two properties inherent to these crystalline materials. The property of molecular sieving may arise from the extremely uniform porosity demonstrated by these crystalline aluminosilicates. The size or shape of one or more components of a mixture may preclude its adsorption by the materials. The separation of n-paraffins from branched chain paraffins is an example of this effect. If a zeolite with a pore opening of .about.5A is employed, the n-paraffin component of a mixture is readily adsorbed, but branched chain paraffins are excluded from adsorption by virtue of their configuration, effecting a separation of the components which is the basis of several commercial processes. If, however, the molecules of the mixture to be separated are all small enough to enter the zeolite crystals, selective adsorption may none the less be demonstrated by a second mechanism. Zeolites have large quantities of exchangeable cations present within their aluminosilicate framework. These cations are situated such that a high proportion may come into contact with adsorbates small enough to enter the crystalline zeolite framework. The energetic interaction of these cations with polar or polarizable adsorbates results in these adsorbates being selectively adsorbed from a mixture of less polar or polarizable species. This effect allows such separations as the selective adsorption of N.sub.2 from air as demonstrated by calcium exchanged A-type zeolite and sodium mordenite by pressure swing or vacuum swing adsorption processes. A comprehensive summary of the adsorptive properties of prior art molecular sieve zeolites, their causes and uses is found in D. W. Breck, Zeolite Molecular Sieves, J. Wiley and Sons, New Yor, Chapter 8, pages 593-724 (1974).
Nitrogen has a quadrupole of 0.31 .ANG..sup.3 and therefore may energetically interact more strongly with the aforementioned cations when O.sub.2, with its quadrupole of only 0.10 .ANG..sup.3. Thermodynamics dictates that the more strongly adsorbed species will be preferentially adsorbed. Further, this cation to N.sub.2 interaction energy, and concomitantly the adsorptive preference or selectivity, may be altered with the choice of exchangeable cations present. In general, in a given zeolite the interaction energy and thus the capacity for nitrogen rises with the charge density of the cation. Thus, it has been found in the literature, Breck, Od. Cit., pages 694-695 and H. Minator and M. Watanabe, Scientific Paper General Education, University of Tokyo, Volume 28, page 218 (1978), that for the monovalent alkali metal cations the following trend of nitrogen capacity exists: Li+&gt;Na+&gt;K+&gt;Rb+&gt;Cs+. Oxygen, with its smaller quadrupole and concomitantly smaller cation-quadrupole interaction energy, is much less sensitive to the cation present. N.sub.2 /O.sub.2 selectivities follow the same trend as N.sub.2 capacities. One would expect that the polyvalent cations would follow a similar trend and due to their high charge density would be even more useful in the separation of nitrogen and oxygen. However, this characteristic has not been clearly demonstrated for any zeolite. In fact, it has been reported that in the faujasite type, e.g., zeolite X, the reverse trend exists as described in U.S. Pat. Nos. 3,140,932 and 3,140,933.
Water, being quite polar, is strongly bound to the aforementioned cations. It has long been recognized that these materials must be activated or dehydrated at elevated temperatures to remove wter or other polar substances, which would block adsorption of such species as N.sub.2. Dehydration of zeolitic adsorbents for gas adsorption generally means that the residual water content in the adsorbent is less than 1.5% by weight water; see Peterson, Zeolites, Vol. 1, pages 105-112 (1981). However, previous zeolite surface scientists in the field of air separation and gas adsorption have completely failed to recognize the sensitivity of the adsorption characteristics of polyvalent-exchanged zeolites in general and calcium-exchanged faujasites in the specific, to thermal activation procedures; see U.S. Pat. Nos. 2,882,224, 3,140,932, and 3,313,091. A good example of this lack of recognition is found in Milton, 2,882,244 which discloses and claims zeolite X adsorbents. It is stated at column 15, lines 23-31 of ths patent that zeolite X may be activated by heating in air, vacuum or other appropriate gases at temperatures as high as about 700.degree. C., at which conditions other adsorbents have been found to be partially or completely destroyed. In fact, a well recognized procedure in the manufacture of such zeolites is to follow the synthesis and/or ion exchange step with a drying step at temperatures of up to about 250.degree. C.
It has not been recognized previously and it has now been found through the use of zeolite content determinations, adsorption measurements, gas chromatographic analyses, and infrared studies which are presented in part in the Examples below, that both cation and framework hydrolysis can give rise to reduced nitrogen capacities and selectivities for the calcium-exchanged X faujasite if the thermal history of the material is not carefully controlled after the ion exchange step has taken place. From these comparisons, it becomes evident that a significant difference in the stability exists between the monovalent and the polyvalent forms of faujasites. The sodium form of faujasite, a common adsorbent used in drying operations, is routinely dried at temperatures of 250.degree. C. without any evidence of a decreased performance as an adsorbent. When one subjects the calcium form of faujasite to the same drying conditions, the resulting adsorbent properties can be inferior to the sodium, but are generally comparable to the sodium form. It is believed that the calcium form of faujasite has been overlooked as an adsorbent for air separation because it was thought not to offer any particular advantage over the sodium form. In fact, out of all of the zeolitic prior art, only one reference suggests that the calcium form has even a slight advantage over the sodium form of faujasite; see Habgood, H. W., Canadian Journal of Chemistry, Vol. 42, pages 2340-2350 (1964). It was found that when what is believed to be the Habgood heat treatment procedures were followed for preparing the X-type zeolites including a highly exchanged calcium form, e.g., greater than 90%, the nitrogen capacity extrapolated to 30.degree. C. and atmospheric pressure was significantly lower than the capacity of the same highly calcium exchanged X-type of zeolite which was prepared under the carefully controlled thermal activation conditions to produce compositions useful in the process of the present invention. Although the specific drying and thermal activation conditions are not set forth in the Habgood reference, it is apparent that the thermal history of his zeolite has not been carefully controlled because of the significantly lower estimated nitrogen capacity and the corrected selectivity values as set forth in detail in the Examples below.
Other references which either have reported gas chromatographic selectivities and/or nitrogen capacities which are significantly lower than those obtained by the absorbent compositions used in the process of the present invention or have disclosed calcium exchanged zeolites without any suggestion as to their having any utility as adsorbents linclude Wolfe, E., et al., German Patent No. 110,478 (1974); Wolfe, F., et al., Z. Chem., Vol. 15, pages 36-37 (1975); Andronikasuili, T. G., et al., Izv. Akad. Nauk. Grvz. U.S.S.R., Ser. Khim., Vol. 1, No. 4, pages 339-402 (1975); Uytterhoeven, J. B., Schoonheydt, R., Liengme, B. V., and Hall, W. K., Journal of Catalytis 13, 425-434 (1969); Ward, J. W., Journal of Catalysis 10, 34-46 (1968); Ward, J. W., Journal of Physical Chemistry 72, 4211 (1968); Olson, D. H., Journal of Physical Chemistry 72, L400-01 (1968); Bennett, J. M. and Smith J. V., Mat. Res. Bull. Vol. 3, 633-642 (1968); Rabo, J., C. Angell, P. Kasai and V. Schomaker, Faraday Society Discussions, Vol. 41, 32 (1965) and Barrer, R. M. and Stuart, W. I., Proceedings of the Royal Society (London) A249, 464-483 (1959).